Question about the vibrations...

About four years ago I started working through the Gateway tapes. I went into it planning to do all of them in order, starting with Orientation. Before I started, I read a lot about them and watched a bunch of videos because my main goal was to eventually have an OBE. At the time I had basically no meditation experience, so everything about it was new to me. The first few tapes were great though, really relaxing and interesting. Then one night while doing Intro to Focus 10 or Focus 12 (can’t remember which), I suddenly felt the vibrations people talk about. It honestly blew my mind. Partly because I assumed deep down that the whole thing might just be imagination, but the sensation was very real and completely unlike anything I had felt before. The problem is I got really excited when it happened and immediately lost focus, and the vibrations stopped. I couldn’t get them back. That was four years ago, and I haven’t experienced them again since. I’ve meditated pretty regularly since then, but nothing even close has happened again. So I’m curious: • What actually causes the vibrations to happen? • Is there a known trigger or state of mind that produces them? • Or is it mostly random/guesswork? For those of you who experience them more consistently, what are you doing mentally when it happens? At this point I mostly just want to experience it again so I know I didn’t imagine the whole thing.